The work of Lotka and Volterra,who arrived independently at several models including those for predator-prey interactions and two- species competition had a profound effect on the field now known as population biology. They were among the first to study the phenomena interacting species by making anumber of simplifyin that led to nontrivial but tractablem a problems. since their pioneering work,many other notable contributions were made. Among these is the work of Kermack and McKendrick (1927),who addressed the problem of out breaks of epidemics in apopulation